Tuning the Work Function of Polar Zinc Oxide Surfaces using Modified Phosphonic Acid Self‐Assembled Monolayers

Zinc oxide (ZnO) is regarded as a promising alternative material for transparent conductive electrodes in optoelectronic devices. However, ZnO suffers from poor chemical stability. ZnO also has a moderate work function (WF), which results in substantial charge injection barriers into common (organic) semiconductors that constitute the active layer in a device. Controlling and tuning the ZnO WF is therefore necessary but challenging. Here, a variety of phosphonic acid based self‐assembled monolayers (SAMs) deposited on ZnO surfaces are investigated. It is demonstrated that they allow the tuning the WF over a wide range of more than 1.5 eV, thus enabling the use of ZnO as both the hole‐injecting and electron‐injecting contact. The modified ZnO surfaces are characterized using a number of complementary techniques, demonstrating that the preparation protocol yields dense, well‐defined molecular monolayers.     

Zinc Induced Aβ16 Aggregation Modeled by Molecular Dynamics

It is widely accepted that the addition of zinc leads to the formation of neurotoxic nonfibrillar aggregates of beta-amyloid peptides Aβ₄₀ and Aβ₄₂ and at the same time destabilizes amyloid fibrils. However, the mechanism of the effect of zinc on beta-amyloid is not fully understood. In this study, a fast zinc-induced aggregation of Aβ₁₆ (as compared to a system without zinc) via the formation of Aβ₁₆ dimers with one zinc ion coordinated in the metal-binding site ₁₁EVHH₁₄, followed by their polymerization, has been studied by molecular dynamics. The best aggregation was shown by the system composed of Aβ₁₆ dimers bound by one zinc ion, with no additional zinc in solution. The presence of Aβ₁₆ dimers was a major condition, sufficient for fast aggregation into larger complexes. It has been shown that the addition of zinc to a system with already formed dimers does not substantially affect the characteristics and rate of aggregation. At the same time, an excessive concentration of zinc at the early stages of the formation of conglomerates can negatively affect aggregation, since in systems where zinc ions occupied the ₁₁EVHH₁₄ coordination center and the His6 residue of every Aβ₁₆ monomer, the aggregation proceeded more slowly and the resulting complexes were not as large as in the zinc-free Aβ system. Thus, this study has shown that the formation of Aβ₁₆ dimers bound through zinc ions at the ₁₁EVHH₁₄ sites of the peptides plays an important role in the formation of neurotoxic non-fibrillar aggregates of beta-amyloid peptide Aβ₁₆. The best energetically favorable structure has been obtained for the complex of two Aβ₁₆ dimers with two zinc ions.     

Electromechanics of Domain Walls in Uniaxial Ferroelectrics

Piezoresponse force microscopy (PFM) is used for investigation of the electromechanical behavior of the head-to-head (H-H) and tail-to-tail (T-T) domain walls on the non-polar surfaces of three uniaxial ferroelectric materials with different crystal structures: LiNbO₃, Pb₅Ge₃O₁₁, and ErMnO₃. It is shown that, contrary to the common expectation that the domain walls should not exhibit any PFM response on the non-polar surface, an out-of-plane deformation of the crystal at the H-H and T-T domain walls occurs even in the absence of the out-of-plane polarization component due to a specific form of the piezoelectric tensor. In spite of their different symmetry, in all studied materials, the dominant contribution comes from the counteracting shear strains on both sides of the H-H and T-T domain walls. The finite element analysis approach that takes into account a contribution of all elements in the piezoelectric tensor, is applicable to any ferroelectric material and can be instrumental for getting a new insight into the coupling between the electromechanical and electronic properties of the charged ferroelectric domain walls.